The use of tungsten powder or pressed tungsten powder compacts in pure or mixtures with other powders under natural conditions in the presence of water and oxygen (e.g., air or dissolved oxygen) leads to the formation of a water-soluble, tungsten-containing species. The first step of the reaction can be described as follows:W+H2O+1.5O2-->WO42−+2H+.
The monotungstate ion, WO42−, reacts with H+, resulting in the formation of the soluble metatungstate anion [H2W12O40]6−:12WO42−+18H+-->[H2W12O40]6−+8H2O.
The formation of this polyoxometalate anion is detectable by its typical UV absorption maximum at 256 nm (molar extinction coefficient, ε256=3.8×104 L(mol·cm)−1).